Abstract: A horizontal cylindrical tank contains a conveyor screw rotatable on a horizontal axis below the symmetry axis of the tank, the latter having at its lower part an inlet at one end for organic material and an outlet at the other end for fermented material. A heating medium is circulated through one of the bearings, a frame supporting the screw, and the latter's peripheral portion. The screw moves sludge along the tank bottom toward the outlet and recirculates fermented material back to the inlet region via a space at one side of the screw, while evolved gas discharges through an outlet in the top of the tank.